1. Field of the Invention
The present invention relates to an ink jet printing apparatus that ejects droplets to print a print medium, and a method of controlling the ink jet printing apparatus.
2. Description of the Related Art
Some ink jet printing apparatuses are of a serial scan type in which printing is performed by a carriage, with a print head mounted thereon, which reciprocates in a direction orthogonal to a print medium conveying direction scanning, while ejecting droplets. In addition to the serial scan type, ink jet printing apparatuses of a full line type are available in which printing is performed by an elongated print head extending over the entire area of the print medium in a width direction thereof and ejecting droplets without scanning. In these ink jet printing apparatuses, the print head and the print medium are spaced from each other, and droplets ejected from the print head cross through the space between the print head and the print medium before impacting the print medium. The ink jet printing apparatuses thus perform printing. The ink jet printing apparatus is generally very quiet, inexpensive to operate, and facilitates size reduction of the apparatus as well as permitting multicolor printing. The ink jet printing apparatus has been widely adopted for printers, copiers, facsimile machines, and the like. Print media onto which droplets are ejected are usually paper media or thin resin sheets (OHP sheets or the like). Furthermore, in recent years, there have been demands for a variety of material as print media. Besides the ordinary print sheets such as paper and thin resin sheets, clothing, leather, and metal have been used for commercial printing apparatuses.
As print media other than paper for printing, thinner print media, for example, thin resin-like print films may be used for the ink jet printing apparatus. In this case, the use of thin print media for printing may result in formation of wrinkles on the print media during conveyance due to the softness of the print media (the lack of elasticity). Furthermore, if roll paper or the like is used for printing, the print media may tend to be bent particularly when the print media are thick and hard. If the print medium with a non-flat print surface is printed using the ink jet printing apparatus, when the print medium is conveyed to a position corresponding to the print head, the print head and the print medium may come into contact with each other. Then, ink attached to the periphery of ejection ports may adhere to the print medium and stain the print medium, or the contact with the print medium may cause the print head to break down. Moreover, the distance between the print head and the print medium may vary, when not in constant. Thus, a position where ink impacts the print medium during printing may deviate from the correct position. This may degrade the quality of images obtained by printing.
A configuration disclosed in Japanese Patent Laid-Open No. 2002-096511 is known as an ink jet printing apparatus that prevents a non-flat print medium from being conveyed to a position corresponding to the print head. In this printing apparatus, a suction port is formed on a holding surface of a conveyance path on which the print medium is held, to suck and hold the print medium. The suction port is connected to a suction fan via a duct. The suction fan is drivingly controlled to suck the print medium onto the holding surface of the conveyance path under a given negative pressure. Consequently, the print medium which wrinkles or tends to be bent is held against the holding surface of the conveyance path of the inkjet printing apparatus. The print medium is thus conveyed in a correct parallel manner. The negative pressure applied to the print media flattens the surface of the print medium to fix the distance between the print head and the print medium. Droplets are thus accurately ejected to predetermined positions on the print medium. As a result, the quality of print images is improved.
When droplets are ejected from an ink jet printing apparatus, droplets (hereinafter referred to as satellites) different from main droplets intended for printing may be generated. Moreover, the satellites may be suspended in the space between the print head and the print medium without coming into contact with the print medium or droplets ejected onto the print medium bounce from the print medium, or the like, to generate fine droplets (hereinafter referred to as mist or ink mist). When the satellites or mist, which is different from the main droplets, is generated simultaneously with generation of the main droplets during printing, the satellites adhere to the print medium to degrade image quality. Furthermore, the mist generated may adhere to a carriage guide shaft or an optical sensor inside the ink jet printing apparatus, which is likely to be corroded to degrade the durability of the ink jet printing apparatus. Thus, droplets other than the main droplets, which are suspended between the print head and the print medium, need to be moved to an area which does not pose a problem or to be collected.
An ink jet printing apparatus including a suction fan to move or collect the ink mist is disclosed in Japanese Patent Laid-Open No. 2006-168187. The suction fan provided in the ink jet printing apparatus generates a given air stream inside the ink jet printing apparatus to move or collect the ink mist. Specifically, Japanese Patent Laid-Open No. 2006-168187 discloses an ink jet printing apparatus provided with a suction fan on the carriage with the print head mounted thereon. The wind speed and air quantity is increased or decreased depending on the number of scans for printing.
The ink jet printing apparatus is drivingly controlled so as to provide a suction pressure and air quantity which satisfy conditions such as temperature and humidity of the printing apparatus and a conveyance length over which the print medium is conveyed as the configuration in which the fan is attached to the ink jet printing apparatus. The fan is generally controlled so as to maintain a given number of revolutions under the above-described conditions.
However, in the ink jet printing apparatus with the fan attached thereto, when the fan is driven, a vibration may be created by the unbalance between blade portions, a variation in motor torque required to rotate the fan, or the like. The vibration occurs as a vibration frequency corresponding to the number of revolutions of the fan.
FIG. 9 shows an example of the waveform of a possible vibration when the fan is driven. The fan is driven at a constant number of revolutions, and a vibration of the frequency corresponding to the number of revolutions occurs in the fan. The vibration caused by the driving of the fan is transmitted via a chassis or main body outer cover of the ink jet printing apparatus to the print head on the carriage or a platen surface on which the print medium is placed for printing. The amplitude and phase of the vibration caused by the driving of the fan in the print head generally differs from those in the platen surface owing to the difference between the transmission path to the print head, located on the carriage, and the transmission path to the platen surface. Therefore, the vibration caused by the driving of the fan may vary the distance between the ejection port and the print medium.
For the print heads in the currently available ink jet printing apparatuses, the density of ejection ports inside the print head is increased to provide high-definition images to be printed. As described above, print heads of a type in which a plurality of ejection ports are formed in one print head are commonly used. However, if a print head with a plurality of ejection ports formed in a main scanning direction is used for printing, the distance between the ejection port and the print medium during ejection of droplets varies among the ejection ports. With reference to FIG. 10, a description will be given of the track of the ejection ports observed when the print head having a plurality of ejection ports formed in the main scanning direction is viewed along the main scanning direction.
FIG. 10 is an enlarged diagram schematically showing the periphery of the print head in order to describe the distance between the ejection port and the print medium observed when droplets are ejected. A print head 501 shown in FIG. 10 is mounted on a carriage 500 and has a plurality of ejection ports arranged in the main scanning direction, in which the carriage 500 performs scanning. For simplification, the print head 501 is assumed to have two ejection ports 502 and 503 arranged at different positions in the main scanning direction. The carriage 500 is attached to a carriage driving belt 504 located so as to extend between a carriage motor pulley 505 and a driven pulley 506. The carriage motor pulley 505 is driven to rotate the driven pulley 506, while moving the carriage driving belt 504 in the main scanning direction. Thus, with the carriage 500 and the print head 501 performing scanning, droplets are ejected to a print medium P through the ejection ports 502 and 503.
Here, if during printing, a vibration created by the driving of the fan is transmitted to the carriage 500, the ejection ports 502 and 503 perform scanning on a track shown in FIG. 9. For simplification, the print medium on the platen does not vibrate but is stationary. The vibration of the print head relative to the print medium and the platen will be considered. In this case, when an area A of the print medium P is to be printed using the ejection ports 502 and 503, the distance between the ejection port and the print medium P varies between when the ejection port 502 reaches a position corresponding to the area A and when the ejection port 503 reaches the position corresponding to the area A. This is because the positions of the ejection ports 502 and 503 in the carriage 500 differ from each other in the main scanning direction, so that when the ejection port reaches the position corresponding to the area A, the phase of the waveform of the track varies between the ejection port 502 and the ejection port 503. In FIG. 10, for a positional relationship between the ejection port and the print medium observed when the print head 501 ejects droplets to the position corresponding to the area A, the distance between the ejection port and the print medium P differs by a distance D. When the difference in distance between the plurality of ejection ports increases, impact accuracy relatively varies between the ejection ports. At the position corresponding to the area A, the distance between the ejection port 502 and the print medium P is longer than that between the ejection port 503 and the print medium P. Thus, in this case, the impact accuracy of droplets from the ejection port 502 is lower than that of droplets from the ejection port 503. This is likely to result in an error when droplets from the ejection port 502 impact the print medium P. Thus, the impact position of the droplets ejected through the ejection port 502 is likely to vary. The impact position of the droplets ejected through the ejection port 502 does not correspond with that of the droplets ejected through the ejection port 503. Thus, when the area A is printed, the resulting image may have a low density.
Furthermore, as shown in FIG. 10, the tracks of the ejection ports 502 and 503 correspond with each other in some areas. Thus, in an area in which the tracks of the ejection ports 502 and 503 correspond with each other, droplets ejected through the ejection ports 502 and 503 impact the same position on the print medium at the same accuracy. As a result, the droplets ejected through the respective ejection ports are likely to correspond with each other. Thus, the insignificant difference in the relative distance from the print medium between the ejection ports 502 and 503 may relatively increase the density of the resulting image. Consequently, the vibration caused by the fan may vary the distance between the ejection port and the print medium observed when the droplets are ejected to the same position through the plurality of ejection ports arranged at the different positions in the main scanning direction. This may vary the density among the resulting images.
In the serial scan ink jet printing apparatus, in which the carriage reciprocates, when the fan continues to be driven at the same number of revolutions, vibration continues at the same frequency. The difference in distance between the ejection ports occurs at the same position in the main scanning direction. Consequently, image density unevenness may occur at particular intervals in the main scanning direction of the print head. Such density unevenness may degrade the quality of images obtained by printing. Multipass printing is sometimes performed in which the same print area is printed by a plurality of carriage scans. As disclosed in Japanese Patent Laid-Open No. 2006-168187, the wind speed of the fan may be reduced for every increase in the number of carriage scans in the same area. However, even the application of such fan control has difficulty avoiding the possible image density unevenness caused by a vibration of a constant frequency resulting from, for example, a variation in motor torque required to rotate the fan.